1. Field of the Invention
This invention pertains generally to gas turbine engine driven powerplants, and more particularly to a gas turbine driven powerplant in which compression of the air is staged, allowing removal of the "heat of compression" from the partially compressed air between stages through the use of an "intercooler." Through the present invention, the heat removed by the intercooler, normally rejected in a cooling tower, and thus lost to the typical intercooled cycle, is recovered and restored to the cycle by means of a unique two-pass high pressure heat exchanger positioned downstream of the final compressor stage. Alternatively, if there is not sufficient intercooler heat to make its recovery important, the unique two-pass high pressure heat exchanger can be used to recover exhaust heat.
2. Description of the Background Art
Gas turbine engines are in wide use, and are ever more often the prime mover of choice. For example, the jet engine is an example of a successful gas turbine application since gas turbine engines have no equal for powering large aircraft. While the modern jet engine is the product of over 50 years of engineering development, the jet engine must fly and, therefore, the design options to enhance performance are necessarily limited. On the other hand, ground applications of advanced aircraft gas turbine engines allow use of additional performance enhancing techniques. In the air or on the ground, however, the principal long term route to increased powerplant performance has been through higher engine compression ratios and higher firing temperatures.
Higher firing temperatures have evolved through a succession of innovative cooling strategies involving compressor bleed air used as turbine coolant to maintain acceptable limits on sustainable temperatures seen by the turbine's metal alloys. Compressor bleed air flow, however, reduces gas turbine power and efficiency. The ever higher compression ratios also necessarily mean higher compressor discharge air temperatures, thereby limiting bleed air cooling effectiveness and requiring additional bleed air flows of the high temperature, high pressure air to effect the same metal cooling. The present invention, as will be shown, will enable metal component cooling with compressor discharge air at or near ambient temperatures. The new coolant, it will be shown, will allow for a significant reduction in bleed air required and/or a significant increase in firing temperature and/or both.
The new coolant will also allow creation of a low temperature heat sink. Low grade heat is normally too cool to be recovered and recycled and, therefore, generally requires continuous removal and rejection to the surroundings by means of a cooling tower or the like. Such heat is generated during operation of ancillary powerplant equipment from sources such as friction in the bearings of the generator and of the gas turbine, and appears in the hot lubricating oil. Low grade heat is also produced by the generator windings in the form of "copper losses," amounting to as much as 0.5 to 1.5 percent of the electrical energy generated, and is generally continuously removed by means of a water cooling loop. Also, the electrical transformer experiences "iron losses" due to the hysteresis effect in the iron core of the transformer. This significant heat load is carried away in the circulating transformer oil, and the heat in the hot oil is rejected to the atmosphere by means of air blast heat exchangers. Economic recovery of this low grade heat for return to the cycle is not possible without a low temperature heat sink which is allowed by this invention.
Another strategy for increasing the power and efficiency of gas turbine powerplants has been to intercool the air between compressor stages. Intercooling allows the high pressure compressor to achieve its designed high pressure air flow with considerably less work, thus increasing efficiency by reducing the deduction of shaft work for compression and allowing more of the shaft work to generate power. One problem with intercooling is that the significant amount of heat extracted from the partially compressed air is difficult to capture and thus most designs assume that this heat will be rejected. Even rejecting this heat, intercooling is an effective strategy for increasing performance. Conventionally, the heat exchange in the intercooler is effected by flowing the hot compressed air across finned tubes counter current to the in-tube flow of cold water. The cooled air from the intercooler is then compressed further before entering the high pressure combustor. The flow of heated water from the intercooler is typically routed to a cooling tower where the heat is rejected to the surroundings and thus lost to the cycle.
Therefore, there is a need for a method and apparatus for recapturing the intercooling heat, returning it to the cycle or producing useful steam and thus providing a substantial additional boost in overall plant efficiency. The present invention satisfies that need, as well as others, and overcomes the deficiencies in prior art powerplant designs.